1. Field of the Invention
The present invention relates to an electrostatic recording apparatus such as a copying machine, a printer, or the like, utilizing an electrophotographic system, and particularly relates to a paper carrying device provided between a transfer portion and fixing device in an electrostatic recording apparatus in which double-surface printing is performed in such a manner that unfixed toner images are formed on the opposite surfaces of paper and fixed by using the fixing device.
2. Description of the Related Art
An electrophotographic system is one of the best known development systems to be utilized in a copying machine or a printer. Particularly in a line printer which requires high-speed printing, ability of higher-speed printing is required with increase of the quantity of information to be processed, and further the picture is required to have high quality and high resolution. Moreover, recently, thinking about saving natural resources or energy is progressed for the purpose of protecting the environment of the earth and hence in the printer, needs for the function of double-surface printing are increased. Here, as a method of double-surface printing by utilizing the electrophotographic system, there is proposed such a method that, in a printer for cut paper, paper is turned over by means of a carrier system constituted by a switchback mechanism after completion of front-surface printing to thereby make printing on the rear surface of the paper. This system, however, is unsuitable for high-speed printing because its paper carrying mechanism is complicated. Further, this system cannot perform printing on continuous paper. Generally, double-surface printing of continuous paper is performed by using two printers. First, paper having one surface already subjected to printing by means of a first printer is turned over by means of a paper turning-over mechanism, so that the other surface of the paper is then subjected to printing by means of a second printer. Although this method is suitable for high-speed printing because the printing speed is never reduced, the volume of the whole printing system is increased because of using two printers to thereby increase the cost of the system. Further, since the toner is separately fixed on the opposite surfaces of paper, two fixing devices using the largest power consumption are required and, as a result, the energy saving is obstructed.
Therefore, a mechanism for fixing toner on the opposite surfaces of paper at the same time has been proposed and investigated. In this proposed mechanism, unfixed toner images are formed on the opposite surfaces of paper by using two sets of photoconductive printing mechanism portions and the toner images are fixed at the same time on the opposite surfaces of the paper by using one fixing device. By this method, high-speed and double-surface printing can be performed with one fixing device.
FIG. 2 is a schematic view showing the configuration of the printer of the double-surface and simultaneous-fixing system. Here, description will be made as to the case of using a photoconductive printing apparatus of the reversal development utilizing a positively charged photoconductive drum of SeTe, As.sub.2 Se.sub.3, Amorphous Si, positively charged OPC, or the like, and positively charged toner. Positive charges are applied from a corona charger 2a to a photoconductive drum 1a; a light pattern is given from an exposure portion 3a such as a laser optical system, LED, or the like, to form an electrostatic latent image; and toner development is performed by means of a developing device 5a. Negative charges are applied from corona transfer device 4a to a rear surface of paper 10 to transfer a toner image 6a on the photoconductive drum 1a to the paper 10. The thus configured photoconductive printing apparatus is defined as a first printing apparatus and a surface of paper to be subjected to printing by this first photoconductive printing apparatus is defined as a front surface. Similarly to this, a positively charged toner image 6b is then transferred to the rear surface of the paper 10 by means of a second photoconductive printing apparatus constituted by a photoconductive drum 1b. The second photoconductive printing apparatus includes similar features to the first photoconductive printing apparatus, such as a corona charger 2b, an exposure portion 3b, a corona transfer device 4b, and a developing device 5b. As a result, toner images 7a and 7b are formed on the opposite surfaces of the paper 10. The toner images 7a and 7b formed on the opposite surfaces of the paper 10 are fused and fixed firmly on the paper by means of fixing device 9 of the heat-roll system. In this case, the reference numeral 8 designates a paper carrying device provided in a paper carrying path between the photoconductive printing apparatuses and the fixing devices so as to change the carrying direction of the paper. As shown in FIG. 3, the paper carrying device 8 is mainly constituted by a paper carrying roll 11 following the paper 10; a cleaning brush 15 for cleaning the paper carrying roll 11; and a charger 13 for applying negative charge to a rear surface of paper. The positively charged unfixed toner 7b is drawn to the paper by means of the charger 13 on one hand and a voltage in a range of from +10000V to +2000V is applied from a high-voltage power source to a core metal of the paper carrying roll 11 on the other hand so that the unfixed toner 7b is prevented from adhering on the surface of the paper carrying roll 11 to the utmost. Further, an electrostatic absorbing force is made to act between the paper 10 and the paper carrying roll 11 so that the paper carrying roll 11 can rotate while following the paper 10 (U.S. patent application Ser. No. 08/925,763 corresponding to JP-A-8-247597). The surface of the paper carrying roll 11 is formed of a conductive material having a volume resistance in a range of from 10.sup.7 .OMEGA..multidot.cm to 10.sup.11 .OMEGA..multidot.cm and toner is prevented from adhering onto the surface by a method of applying a voltage to the core metal. Therefore, it is not necessary to provide any external charger for charging the paper carrying roll, for example, as described in JP-A-7-072776. Adhering matters 14, particularly toner, collected by the cleaning brush 15 are knocked down by means of a knocking rod 16 and collected into a discharged-toner collecting box 17.
Here, in the case where a sudden change of the speed, for example, paper jamming occurred, it was found that, as shown in FIG. 3, the toner 7b transferred onto the rear surface of the paper 10 fell onto the paper carrying roll 11 to form a contaminant 7c on the surface of the paper carrying roll 11. If the contaminant 7c would adhere, as shown in FIG. 4, on the downstream side with respect to the cleaning brush 15 and before the contacting position of the paper 10 with the paper carrying roll 11 upon stoppage of printing, the contaminant 7c would move from the surface of the paper carrying roll 11 to the rear surface of the paper 10 after re-starting of printing to thereby lower the picture quality. The adhesion of the contaminant 7c is a phenomenon which can be recognized also in the case where printing has not been performed for a long time regardless of paper jamming. In this case, dust in the air or toner or paper powder floating in the apparatus causes the phenomenon. Consequently, it is necessary to provide means for removing dust, toner, or paper powder before starting of printing again. The background-art paper carrying roll 11 (JP-A-8-247597) could not remove the contaminant 7c adhering on the surface of the paper carrying roll 11 upon stoppage of the printing because the paper carrying roll 11 followed the paper. Therefore, when printing was started again, paper was wasted by at least a half-circumferential part thereof.